1. Field of the Invention
The present invention relates to a quartz crystal unit for use at high frequencies of, for example, 100 MHz or more, and more particularly to a crystal unit having excellent secular change characteristics and oscillation characteristics, and to a holding structure for the crystal unit.
2. Description of the Related Art
Crystal units in which a quartz crystal blank is accommodated inside a receptacle are incorporated in oscillators as frequency control devices, and particularly, as the reference for communication frequencies. With the increasing use of optical communication in recent years, crystal units with higher oscillation frequencies are now in demand.
In a crystal unit that employs an AT-cut quartz crystal blank, which is a representative crystal blank, the resonance frequency is inversely proportional to the thickness of the crystal blank. To obtain a crystal unit having higher oscillation frequencies, crystal units are being developed in which a depression is provided in the vibration region of the crystal blank and the thickness of the crystal blank in this depression decreased, whereby not only is the oscillation frequency raised, but the vibration region is held and the mechanical strength maintained by the relatively thick portions around the periphery of the depression. This type of crystal blank is disclosed in, for example, U.S. patent application Publication Ser. No. 2004/0135471 A1.
FIGS. 1A and 1B are a plan view and a sectional view showing one example of a crystal unit of the prior art, respectively. FIG. 1A is a plan view showing a state in which cover 10 of the crystal unit has been removed.
The crystal unit is provided with rectangular AT-cut crystal blank 1, crystal blank 1 being accommodated inside receptacle body 2. A circular depression is formed in substantially the central portion of one principle surface of crystal blank 1, and the crystal blank is thinner in the bottom surface of the depression than at the outer periphery, this thin portion being vibration region 3. A pair of substantially circular excitation electrodes 4a and 4b is formed on the both principle surfaces of crystal blank 1 in vibration region 3, respectively. Extending electrodes 5a and 5b are provided on the respective principle surface so as to extend from corresponding excitation electrodes 4a and 4b toward the two opposing ends of crystal blank 1. Each of extending electrodes 5a and 5b connects to a corresponding excitation electrode over approximately half of the outer circumference of the excitation electrode and extends to the outer periphery of crystal blank 1 by way of a fan-like expanding region. This expansion of the connection region between the excitation electrodes and extending electrodes reduces the electrical conductive resistance between the excitation electrodes and the extending electrodes to a low level.
Receptacle body 2 has a depression and is composed of laminated ceramics. A pair of internal terminals 6a and 6b composed of a thick-film metal are formed on the inside bottom surface of receptacle body 2, and internal terminals 6a and 6b are electrically connected to a pair of external terminals 7a and 7b for surface mounting that are provided on the outer surface of receptacle body 2.
Crystal blank 1 is electrically and mechanically connected to receptacle body 2 by securing one end of the crystal blank to which extending electrode 5b is extended on one internal terminal 6a that is provided on the inside bottom surface of receptacle main body 2 by means of eutectic alloy 8. In addition, the other end of crystal blank 1 to which extending electrode 5a is extended is electrically connected to the other internal terminal 6b by wire bonding that uses gold wire 11.
In order to prevent the other end of crystal blank 1 from bending upon wire bonding, the other end of crystal blank 1 is placed on pillow member 9 provided on the inside bottom surface of receptacle body 2. After wire bonding has been completed, the open face of receptacle body 2 is covered by cover 10 to hermetically seal crystal blank 1 inside the receptacle and thus complete crystal unit.
In the crystal unit according to the foregoing description, only one end of crystal blank 1 is secured to receptacle body 2 by eutectic alloy 8, and crystal blank 1 oscillates with this secured end as the fixed end. The other end of crystal blank 1 functions as a free end even though this end is placed on pillow member 9 and connected to gold wire 11 of wire bonding. In this configuration, the securing of crystal blank 1 at only one point prevents the occurrence of distortion caused by the difference in thermal expansion between crystal blank 1 and receptacle body 2, and further, maintains an excellent frequency-temperature characteristic of the crystal unit.
However, in a crystal unit of the above-described configuration, a pillow member 9 is necessary for carrying out wire bonding at the other end of crystal blank 1, and this other end therefore contacts pillow member 9 when crystal blank 1 is oscillating. In other words, the other end of crystal blank 1 is not a completely free end, and the possibility therefore exists for difficulty in initiating vibration at the crystal unit as well as for a deterioration in the characteristics of the crystal unit.
To eliminate this possibility of deterioration, pillow member 9 is preferably removed after carrying out wire bonding, but pillow member 9 is difficult to remove from inside the depression of receptacle body 2 after securing crystal blank 1. This removal becomes particularly difficult with increased miniaturization of the crystal unit. In addition, even if pillow member 9 can be removed, there is the problem of an increased number of fabrication steps.